1. Field of the Invention
The present invention relates to a method for preparing a copper-containing catalyst usable in a hydrogenation reaction, and to a method for producing an alcohol. More specifically, it relates to a method for preparing a copper-containing catalyst usable in hydrogenation reaction (hereinafter referred to as a copper-containing hydrogenation reaction catalyst) by a liquid phase reduction under specified temperature conditions, and to a method for producing an alcohol of high quality at a high productivity using the copper-containing hydrogenation reaction catalyst which is prepared by the above method and which has markedly improved catalytic activity and selectivity.
2. Discussion of the Related Art
Since the 1930s a number of methods have been disclosed for producing aliphatic alcohols, alicyclic alcohols or aromatic alcohols by hydrogenating carboxylic acids or esters of carboxylic acids. In those methods, copper catalysts are mainly proposed for use in hydrogenation of esters of carboxylic acids, particularly fatty acid esters and copper-chromium catalysts are commonly used for industrial purposes.
Determination of the conditions employed to activate the precursors of these catalysts by reduction depends on the form and reduction method of the precursors, usage of the obtained catalysts, and other factors. For example, when the fluidized bed reaction system is used, a powder form is employed. In Japanese Patent Laid-Open Nos. 1-305042, 5-177140 and 5-117185, it is stated that a catalyst precursor may be activated by gas phase reduction or by liquid phase reduction in a solvent exemplified by hydrocarbons such as paraffin, ethers such as dioxane, alcohols, and esters. Gas phase reduction, however, requires an additional apparatus other than the reactor for reductive activation of a powdery catalyst precursor and further needs a surface stabilizing treatment for preventing the resulting copper from being oxidized by air. Because of these drawbacks of gas phase reduction, liquid phase reduction is generally employed in the fluidized bed reaction system. In this case, it is generally agreed that reduction is carried out preferably at a temperature of from 150.degree. to 350.degree. C. until hydrogen absorption has stopped. Since heat removal is easy in the case of a powdery form, local overheating can easily be prevented.
On the other hand, when a fixed bed reaction system is used, gas phase reduction is exclusively used for the reductive activation of a formed catalyst precursor, and, for industrial purposes, it is common practice to carefully reduce a catalyst precursor at a given temperature while supplying an inert gas containing several to several dozens percents of hydrogen, to prevent local overheating due to rapid reduction.
Reduction of copper oxide with hydrogen is generally known to generate a heat of reduction of 20 Kcal per mole of copper oxide and the reduced copper thus obtained has a very low thermal stability. For this reason, it is important to gradually reduce the copper oxide while controlling heat generation to prevent the deterioration of catalyst performance. When using a formed catalyst precursor, in particular, this is critical because heat removal is difficult.
It is, therefore, very liable that when a catalyst precursor is activated by gas phase reduction with a high concentration of hydrogen in a short time, a rapid heat generation considerably degrades catalyst performance, and that when a large amount of catalyst precursor is activated by reduction in a short time on an industrial scale, a rapid rise in temperature causes a very dangerous situation. For this reason, it is common practice to use a low concentration of hydrogen over a long period of time for activation of catalyst precursors containing copper oxide by gas phase reduction. For example, Japanese Patent Laid-Open No. 61-161146 states that it takes as long as 4 to 14 days for catalytic activation by such reduction, indicating a disadvantage of gas phase reduction in view of alcohol productivity.
Also, DT 1768313 discloses a method for reductive activation of a copper-zinc oxide catalyst precursor, in which the catalyst precursor is gradually reduced at a temperature of between 120.degree. and 240.degree. C. in a hydrogen-containing nitrogen gas stream and finally treated with high-pressure hydrogen at a temperature of from 250.degree. to 300.degree. C. for 1 to 2 hours. Japanese Patent Laid-Open No. 62-298457 states that a copper-chromium oxide catalyst precursor can be activated by raising a temperature from 130.degree. C. to 200.degree. C. at a rate of 10.degree. C./hr and keeping it at 200.degree. C. for 12 hours in a nitrogen gas stream containing 1% by volume hydrogen. Also, DE 3443277A1 discloses a method for reductive activation of a copper-zinc oxide catalyst precursor, in which the catalyst precursor is reduced at 200.degree. C. in a nitrogen gas stream containing 5% by volume hydrogen for 16 hours and then further reduced with pure hydrogen at 200.degree. C. for 16 hours. Japanese Patent Laid-Open No. 61-178037 states that a copper oxide-magnesium silicate catalyst precursor can be activated by reducing at 200.degree. C. in a nitrogen gas stream containing 1 to 2% by volume hydrogen for 60 hours. In addition, Japanese Patent Laid-Open No. 1-127042, which discloses a method for reductive activation of copper-chromium oxide and reviews the prior arts, indicates that all methods require reduction temperatures of not lower than 150.degree. C. for catalyst precursor activation.
Although gas phase reduction is commonly used in the fixed bed reaction system, several methods of liquid phase reduction are also known to activate a catalyst precursor containing copper oxide. For example, Japanese Patent Laid-Open Nos. 5-177140 and 5-117185 propose to activate a copper-zinc oxide catalyst precursor at 200.degree. C. in an autoclave by a batch reaction method in liquid phase. Also, British Patent Publication No. 385625 describes a method of liquid phase reduction of a copper-chromium catalyst precursor at 325.degree. C. in an ester flow of a liquid hourly space velocity of 8.0 in the fixed bed reaction system, followed by hydrogenation of the ester. Also, Japanese Patent Laid-Open No. 47-14113 discloses a method of liquid phase reduction of a precursor of copper-chromium catalyst at 200.degree. C. in a lactone flow of a liquid hourly space velocity of 0.67 in the fixed bed reaction system, followed by hydrogenation of the lactone. According to Japanese Patent Laid-Open No. 2-26611, the reduction of a catalyst precursor containing copper oxide can be carried out after an ester, the starting material, has been supplied.
However, all these activation methods by liquid phase reduction have practically no advantages over those by gas phase reduction, as Japanese Patent Laid-Open No. 2-26611 states that "reduction of the catalyst's copper component by these methods is not complete and somewhat difficult to control".
According to the findings of the inventors, reductive activation with hydrogen at a temperature of 150.degree. C. or higher under the stream of an ester or an alcohol has drawbacks as mentioned below.
When an ester is used as the solvent, hydrolysis of ester with water produced upon catalyst reduction occurs to produce fatty acids, catalyst poisons to copper-containing catalysts. Therefore, such esters cannot directly be used as a starting material for alcohol production. Also, fatty acids formed during the reductive activation of a catalyst precursor cause dissolution of copper and other problems leading to deterioration of the catalyst. When an alcohol is used as a solvent, as the reductive activation of a catalyst proceeds, an ester wax is formed from two alcohol molecules and causes a significant decrease in the purity of alcohol. In addition, the higher the temperature for activation is, the more hydrocarbons due to alcohol decomposition are formed, which also significantly decrease the purity of alcohol.